Coastal beach erosion is defined by actual removal of sand or other soil from a beach and depositing it offshore, inland or along tidal bays or estuaries. Such erosion can result from normal tidal action, severe wave action arising from storms and hurricanes, and from simple inundation due to rising water levels. The capacity of a Newtonian fluid, such as water, to carry solids is dependent upon the velocity and the density of the fluid. The density of water is related to the composition and amount of dissolved solids. Seawater for example will have salt and other solid particles carried in suspension. The size of suspended particles can range from microscopic to sand grains and on up to parts of sea shells and other organic matter.
The primary mechanisms responsible for erosion or accretion of sand/sediment are bottom currents flowing within a few inches up to as much as a foot or more above the shore bottom. These bottom currents act on the sand or sediment in a way that depends on the velocity of the currents and the force of the water against the sand, sediment or rocks situated along the shore. The bottom features (such as slope, smoothness, and undulations) affect the degree of packing of the sediment and thus has a large effect on the ability of current flow to erode or deposit sediment onto the bottom.
Water currents are produced by wind, tidal action, and variations in temperature, salinity or turbidity. Man-made coastal inlet, in particular ones that have hardened and have dredged channels tend to shunt inland or estuary water offshore. When this outflow of fresh and brackish water from land and estuaries combines with tidal flows into and out of the estuaries and bays into the gulfs or seas, it can cause instability in the water flow along the near-shore on both sides of inlets that can extend for great distances along the coast line.
If the predominant flow of water is outward from the coastal inlets this action will pull or suction water along the coastline and into the channel flow of an inlet. Gravitational effects causing current oscillations in and out of the inlets will increase the flow of water parallel to the shoreline. Bottom currents, generated by this flow of water to the inlets, are generated further offshore and typically run parallel to the shoreline between the beach and an offshore sandbar. These offshore, long-shore currents differ from classical near long-shore currents, normally identified as littoral currents that carry sand into the shore at an angle (depending upon the wind), but also carry sand out, perpendicular to the shoreline. These near-shore long-shore currents can be very strong and fast. They can move sand along the beach at several miles per hour. Wind-generated waves will also cause erosion or accretion of sand onto the beach or near-shore environments. Long-shore currents are one of the principal factors for the movement of sand into, along or from the beach and near-shore areas. To differentiate these two currents that act parallel to the shore line, we will stick to the term of “near-long-shore” currents that are near shore, normally very visible on the beach and refer to “offshore-long-shore currents” that are generated by inlets, currents which are present a distance from the beach, between the beach and an offshore sand bar.
The most common and well-known method for beach restoration is depositing sand dredged from a nearby offshore location. Less often, sand can also be excavated onshore and hauled overland to the point where it is deposited onto the beach, (see: Method of building or restoring marshes and beaches U.S. Pat. No. 4,759,664 and others). This restoration method was designed not only to build up beach sand, but also to construct a shallow-slope profile away from the beach to a point as far offshore as possible. An advantage of having a shallow-slope beach profile is that the wave energy and erosional power, particular storm event waves, are dissipated resulting in less beach erosion. However, this method of beach restoration has proven to be somewhat temporary and very costly. It can also be harmful to the environment by having a negative effect on the benthonic life on the bottom. In many places it has been effective for only a short period of time which requires it to be repeated periodically and especially after a major storm event.
Some of the more common and well-known methods of beach erosion control are barrier walls, concrete riprap, and concrete revetments. These structures are normally installed parallel to the beach. Despite being unnatural and eyesores, such structures have been successful in preserving or protecting property on the landside of the structure, however, they often accelerate the beach sand erosion process due to the fact that the full force of the incoming wave energy is not dissipated, particularly storm waves. The velocity of water energy contained in the wave does not allow time for the sand to drop out under the force of gravity and accrete on the beach. These structures can also have a negative impact by causing erosion on adjacent beaches. Construction permits for such structures are difficult to obtain where they may impact sea turtle nesting grounds.
Other erosion control devices include “groins.” These structures normally extend into the water and perpendicular from the beach and are designed for high frequency, low magnitude storms. Many times they are used in tandem with seawalls. Groin structures are anchored into or lie on the seabed and can extend considerable distances into the water. They can be submerged or their tops can appear above the water surface. Structures in this category include T-groins, permeable groins, Longard Tubes (geo-tubes), jetties, seawalls, and bulkheads. These devices are designed to trap sand on the upside of the direction of the current while diverting the entire water column, current energy and excess sand to deeper water further offshore. In other words, sand is accreted on the upside beaches, and downside beaches become deprived of their sand supply. To be effective multiple groins must be placed along the entire shoreline at intervals of several hundred feet. The accretion process must be jump-started by filling in the front of each structure with sand. The disadvantage of groins is they are unnatural eyesores, they can be dangerous to surfers and swimmers and they can have a negative impact on turtles and other sea life. Plastic mesh groins are not durable and can be subject to vandalism.
Geosynthetic textile tubes can be made out of a variety of specialized, strong, UV and water-resistant synthetic fabrics. The material can be one tube or a series of tubes with different diameters sewn together. The tubes can be filled with sand or concrete once placed on the sea bed. Geo-textile tubes are disadvantageous because they are often expensive eyesores that are time consuming and difficult to install. They are subject to breakage and vandalism. Using sand fill involves dredging from a nearby source or truck-hauling to the fill site. Using concrete fill makes the geo-textile tubes more durable, but also rigid, permanent fixtures that become very expensive to remove if they did not work. Being rigid structures they may break during high magnitude storm events and thus less effective and may become an environmental hazard.